The quality of a rear projection system is determined by several parameters which are described in relation to the user reaction. Several of these parameters are dependent upon the properties of the screen itself either in conjunction with or independent from the rest of the system. Screen fall-off refers to a decrease in the screen brightness at positions to the side of the screen center. This arises from a combination of non-uniformity of the projection system and poor distribution of the light scattered by the screen. Screen resolution relates to the detail visible on the screen independent of the resolution of the projection system. Screen breakthrough is manifested by a bright spot in the screen due to light which is not diffused.
Another effect which is not as easily defined but contributes greatly to eye fatigue is speckle. This is the occurrence of multitudes of tiny bright spots in a light area of the image which seem to dance or scintillate as the viewer moves. This is particularly bothersome for users who must observe a screen for long periods of time, for example, parts catalogs recently have been put on film and are viewed on screens for periods of time ranging up to eight hours a day. Also, instructional speed reading devices employing rear projection screens often are viewed by students for periods of time ranging up to two hours.
In one commercially available rear projection screen speckle is eliminated by employing two screens separated by a few mils. One of the screens is kept in constant orbital motion relative to the other screen by a motor driven mechanism. This rather elaborate motional relationship is effective in reducing speckle but adds considerable expense to the system.
In another type of speckle eliminating screen, an organic compound, such as a liquid crystal, is employed. When the electric field across the compound is beyond a threshold value, the compound undergoes turbulent motion, the optical properties of the compound change dynamically and that change is manifested as a rapid change in the scattering of incident light. That scattering change is effective to eliminate some speckle but once again the system is rather elaborate and expensive and long device lifetimes have not been demonstrated.